Dense wavelength division multiplexing (DWDM) increases the capacity of embedded fiber by assigning incoming optical signals to specific frequencies (wavelength, lambda) within a designated frequency band and then multiplexing the resulting signals out onto one fiber. DWDM combines multiple optical signals so that they can be amplified as a group and transported over a single fiber to increase capacity of the telecommunication network. Each signal carried can be at a different rate (OC-3/12/24, etc.) and in a different format (SONET, ATM, data, etc.). Limiting bandwidth of the useable band of the optical fiber to accommodate future growth is the driving force behind the effort to increase the spectral efficiency of DWDM systems.
FIG. 1 is a block diagram of a prior art simplex DWDM system. A DWDM multiplexer 110 combines several optical signals, hereinafter referred to as channels, into a single multi-channel optical signal that is transmitted through the optical fiber 120. Optical amplifiers 125 may be connected to the optical fiber 120 to amplify the optical signal. Conversely, the DWDM demultiplexer 130 receives the multi-channel optical signal transmitted through the optical fiber 120 and splits it into separate channels. Each channel is characterized by a distinct wavelength designated as λi in FIG. 1 where the index, i, runs from 1 to N where N is the number of channels in the DWDM system. It is understood that a wavelength has a corresponding frequency fi and that one may refer to either frequency or wavelength while meaning the same physical attribute of the signal. For an N-channel DWDM system, there are N transmitters 140 and N receivers 150 with one transmitter 140 and one receiver 150 for each channel. A transmitter 140 generates the optical carrier signal at the channel wavelength and modulates the carrier signal with a single data stream before transmitting the modulated optical signal to the multiplexer 110. The multiplexer 110 then combines the N modulated optical signals having different channel wavelengths into a single multi-channel optical signal, and sends this through the fiber 120. The demultiplexer 130 receives the multi-channel optical signal and separates it into the different channel wavelengths. Each receiver 150 then demodulates one of the demultiplexed channel signals to extract the data signal. While FIG. 1 shows a prior art simple system, it is understood that in real life, a duplex system is used, with one or more transmitters and receivers at each end. Dutton, Harry J. R., Understanding Optical Communications, 1998, pp. 513–568, ISBN 0-13-020141-3 presents a description of the DWDM system and of its components and is herein incorporated by reference.
The data rate (in bits per second or bps) through a single optical fiber may be increased by combining one or more of the following methods: increasing the data modulation rate; increasing the number of channels per fiber; and selecting a modulation method having a higher spectral efficiency.
Increasing the data modulation rate is limited by semiconductor technology and cost, as well as frequency-dependent fiber impairments as chromatic and Polarization Mode Dispersion (PMD). Increasing the number of channels per fiber is limited by the properties of optical component materials. Current and proposed implementations of DWDM systems use a channel modulation rate of about 10 GHz (OC-192) and use 40 channels over the conventional optical band (C-band) between 1530 nm and 1560 nm. Therefore, the transmission bit-rate through a single optical fiber is about 400 Gbps. Each channel has a bandwidth of about 100 GHz. The spectral efficiency is defined as the channel bit-rate divided by the channel bandwidth. The spectral efficiency of the system is therefore 0.1 bit/Hz. The spectral efficiency may be doubled by using a coherent modulation technique such as quadrature phase shift keying (QPSK). QPSK encodes two bits per modulation period and therefore doubles the channel transmission bit-rate to 20 Gbps. The two bits encoded during QPSK are referred to as a symbol and the modulation period is referred to as the symbol period. The inverse of the symbol period is the symbol rate.
The channel bit-rate may also be doubled by combining two data streams into a single channel. U.S. Pat. No. 6,038,357 issued to Pan discloses a fiber optic network that combines two data streams into a single channel by polarizing the optical signal modulated by the first data stream to a polarization plane that is orthogonal to the polarization plane of the optical signal modulated by the second data stream.
Polarization mode dispersion (PMD) arises in optical fiber when circular symmetry is broken by the presence of an elliptical core or by noncircularly symmetric stresses. The loss of circular symmetry results in the difference in the group velocities associated with the two polarization modes of the fiber. The main effect of the PMD is the splitting of the narrow-band pulse into two orthogonally polarized pulses (dual imaging) that propagate through the fiber with the different group velocities. As the dual images propagate through the birefringent fiber, there states of polarization (SOP) constantly undergo changes causing the random coupling between the two images.
The PMD varies randomly from fiber to fiber. In the single fiber, the PMD also varies randomly with the optical carrier frequency and ambient temperature. PMD broadens and degrades the signal and limits the distance the signal may propagate before the information encoded in the signal is lost.
Therefore, there remains a need to improve the spectral efficiency of existing/planned DWDM standards (OC-48 at 2.048 Gbps or OC-192 at 10 Gbps) using existing fiber optic cables. There also remains a need for PMD compensation of the received optical signal.